1. Field of the Invention
The present invention relates to a receiver circuit used for a communication apparatus, more particularly to a receiver circuit adapted to communication apparatuses operative with different power voltage ranges.
2. Description of the Related Art
Conventionally, a vehicle control system adopts a communication system in which a plurality of communication apparatuses connected each other via communication line. In the communication system, a communication data is exchangeable among the communication apparatuses via the communication line. The communication data received or transmitted via the communication line is a voltage signal generated from a direct current (referred to DC) power source of which output voltage may vary.
Specifically, vehicles implement various ECUs (electronic control units) mutually connected via a communication bus as a communication line, which configures the communication system in the vehicle. This configuration is commonly used for a vehicle communication system. Specifically, Japanese Patent Application Laid-Open Publication No. 2005-335607 and 2009-526451 disclose a communication system in which the communication is performed by using a two states voltage signal, consisting of high level referenced to an output voltage of a battery mounted on the vehicle, and low level referenced to the ground potential (GND=0V).
In the above-described communication system adapted to the vehicle communication system, respective ECUs recognize either high or low level on the communication bus based on a threshold voltage. Generally, the ECUs mounted on the vehicle are adapted to operate with power supplied by the on-vehicle battery, i.e., battery voltage is supplied to the ECUs. Therefore, the threshold voltage can be generated by a voltage divided from the battery voltage. Specifically, a comparator is used to compare the voltage signal from the communication bus with a voltage divided from the battery voltage Vbat, the voltage being divided into one half of the battery voltage Vbat with two resistors having the same resistor value. Hence, the comparator outputs a two state signal as a reception signal based on the voltage signal on the communication bus.
Alternatively, one of ECUs mounted on the vehicle may be supplied a constant voltage generated by a constant voltage source as a power supply instead of the battery. Assuming this type of ECU exists in the communication system mixed with the ECUs using the battery as a power source, it is possible that stable communication may be disrupted.
Specifically, as shown in FIG. 1, the ECU is supplied with a constant voltage generated by a constant voltage source. The receiver is arranged in the ECU, which is configured as a voltage divider and a comparator. Thus, the ECU operated by the constant voltage as the power source uses the threshold voltage generated by dividing the constant voltage so that the threshold voltage is kept constant regardless of the variation of the battery voltage. As a result, when the variation of the battery becomes larger, the voltage signal of the communication bus cannot be determined correctly. For instance, when the specification of the communication bus is defined such that the bit ‘1’ is defined as voltage equal to or more than 60% of the battery voltage, and the bit ‘0’ is defined as voltage equal to or less than the battery voltage, if the battery voltage varies from 8V to 16V, the threshold value should be set to 3.2V to 4.8V when the battery voltage is 8V, and set to 6.4V to 9.6V when the battery voltage is 16V. Accordingly, a fixed value cannot be used as the threshold value.
The above-described problem occurs not only in a communication system which is mounted on the vehicle, but occurs in a communication system configured to generate the voltage signal used for the communication line, the voltage signal being generated from the DC power supply in which the output voltage varies.